Collagen is an abundant structural protein that is present in all animals. Collagen is insoluble in water, but when properly prepared, can hold and retain many times its own mass in water due to its naturally charged surface characteristics. In fact, the surface charge of collagen is a key physical property that allows the protein to be adapted for a wide variety of practical uses including environmental remediation, purification of biological samples, and other engineering and biotechnological applications.
Dispersions of collagen have been formed using a starting material of raw fibrillar type I bovine corium. See U.S. Pat. No. 6,660,829. It was found that collagen dispersions can be frozen and then freeze dried, resulting in a product that retains the overall dimensions of the original frozen material. At the same time, over 99% of the volume of the lyophilized product is porous “empty” space and the remaining protein component has a spongy organic aerogel-type structure with controllable pore size, good mechanical properties, and a density as low as one thousandth of water. It was further found that this solid material can be crosslinked to anchor or memorize its shape, pore size and morphology. Id.
It was also disclosed that such dispersions can be used in environmental applications, and may function as an aid to filtration, to assist in the separation of pollutants (including metals and soluble organic molecules) from aqueous streams, for selective fractionation of molecules, and in oil droplet stabilization. Variations on the production process for dispersions of this type could provide additional end products and useful results not heretofore observed.
Porous metal or ceramic materials are useful for the fabrication of devices such as filters, heat exchangers, sound absorbers, electrochemical cathodes, fuel cells, catalyst supports, stand-alone catalysts, fluid treatment units, lightweight structures and biomaterials. The end use of the porous material may be determinative of the requirements for structure (such as open/closed porosity, pore size distribution and shape, density) and physical properties (such as permeability, thermal, electrochemical and mechanical properties). For example, closed porosity is usually sought for lightweight structures, while open porosity may be desired when one or more of surface exchange, permeability, and pore connectivity is required.
Numerous different approaches exist for the fabrication of such porous materials. Deposition techniques have been used for the fabrication of metal foam. U.S. Pat. No. 4,251,603 and Japanese Patent Application No. 5-6763 describe processes that involve plating a sponge-like resin followed by burning the resin to obtain a metal foam. Deposition may also be performed from salts (U.S. Pat. No. 5,296,261) or gas (U.S. Pat. No. 4,957,543). Those processes provide low-density materials having open-cell porosity.
Techniques involving the deposition of powders on polymer medium (foams or granules) have also been developed. Those techniques consist in deposing metal or ceramic particles on a polymer and burning the polymer to obtain porous metal or ceramic materials. U.S. Pat. No. 5,640,669 describes a process for preparing a metal porous body having a three-dimensional network structure by deposing a layer comprising Cu, a Cu alloy, or a precursor thereof on a skeleton composed of a porous resin body having a three-dimensional network, followed by heat-treating the resin body with the layer formed thereon to remove the heat-decomposable organic component, thereby forming a porous metal skeleton of Cu or a Cu alloy.
U.S. Pat. No. 5,759,400 describes the fabrication of metal foams by cutting a polyethylene foam to form a substrate having a desired size and shape, submerging the polyethylene substrate into a solvent for a period of time effective to provide a substrate with a tacky surface, coating the tacky surface of the polyethylene with a slurry of copper powders admixed with a binder, drying the impregnated polyethylene foam, burning the polyethylene in a furnace to produce a foam structure consisting of copper and sintering the final product to obtain a rigid structure.
U.S. Pat. No. 5,881,353 discloses a method for producing a porous body with high porosity by coating a resin foam, such as urethane foam, with an adhesive to impart stickiness to the surface of the foam, and thereafter a powder such as copper oxide powder is applied thereto, followed by heating to remove the substrate and sinter the powder. Thus, a porous body to which the pattern of the base material has been transferred is produced. The powder may be appropriately selected to obtain porous bodies having a great strength, without limitations on materials.
Methods for preparing porous hollow spheres and sponge like particles are described in U.S. Pat. No. 4,775,598. Such porous hollow spheres could be used to produce porous materials. The process for making hollow spherical particles comprises the steps of providing metallized lightweight spherical bodies from cores of a foamed polymer with a metal coating of a thickness of 5 to 20 microns; coating said metallized lightweight spherical bodies with a dispersion of at least one particulate material selected from the group which consists of metals, metal oxides, ceramics and refractories to a dispersion coating thickness of 15 to 500 microns; drying the dispersion coating on said metallized lightweight spherical bodies to form a dry layer of said material thereon; heating said metallized lightweight spherical bodies with said dry layer of said material thereon to a temperature of about 400° C. to decompose said polymer cores and form hollow bodies essentially consisting of said metal coatings and said dry layers of said material thereon; and subjecting said hollow bodies essentially consisting of said metal coatings and said dry layers of said material thereon to a sintering temperature of 900° C. to 1400° C. for a period sufficient to sinter the material of the respective layer and the respective layer to the respective metallic coating, thereby forming hollow spherical particles.
In view of the numerous end uses for porous metal or ceramic materials, there remains a need for structures that are characterized by high porosity, strength, durability, and uniform pore distribution, and that can be made by methods that allow for the adjustment of pore size, porosity, or other characteristics of the final product, depending on the desired end use.